This application claims priority of the German patent application 102 22 041.7 filed May 10, 2002, which is incorporated by reference herein.
1. Field of the Invention
The invention concerns an afocal zoom for use in microscopes having a tube lens, the zoom comprising four successive optical assemblies when viewed from the object end, the first assembly having a positive focal length, the second assembly a negative focal length, the third assembly a positive focal length, and the fourth assembly a negative focal length, and the first and the fourth assembly being arranged in stationary fashion and the second and the third assembly being arranged movably for modifying the magnification of the zoom, the zoom magnification decreasing with increasing distance between the second and the third assembly. The invention furthermore concerns a microscope as well as a stereomicroscope having such an afocal zoom.
2. Description of the Related Art
Microscopes, in particular stereomicroscopes, having an afocal zoom of the aforesaid kind are used wherever high specimen magnification is required, for example in technology enterprises for the manipulation and inspection of small objects, e.g. semiconductor features or micromechanical objects; in research institutions in the biological sciences and materials science; and, for example, for the examination and manipulation of cells or even for surgical purposes. As miniaturization continues and as ever-smaller specimens are being investigated, not only do the requirements concerning the resolution of such microscopes increase, but the size of the field of view at low magnification (for rapid positioning of specimens and for an improved overview during inspections) also becomes more important.
In order to increase a microscope""s magnification and allow it to be modified steplessly over a certain range, the microscope is equipped with a zoom. An afocal zoom images an object at infinity in an image located at infinity. Designating the angle with respect to the optical axis at which an object point appears at infinity as wE, and the emergence angle (after passing through the zoom) at which the image point appears at infinity as wA, the magnification of the zoom is then VZO=tan(wA)/tan(wE). The zoom system allows magnification to be varied without changing the location of the object or the image. The ratio between maximum and minimum zoom magnification is called the xe2x80x9czoom factorxe2x80x9d z.
FIG. 1 shows an afocal zoom 1 constructed in accordance with the preamble of Claim 1. A zoom construction of this kind is known, for example, from xe2x80x9cOptical Designs for Stereomicroscopes,xe2x80x9d K. -P. Zimmer, in International Optical Design Conference 1998, Proceedings of SPIE, Vol. 3482, pp. 690-697 (1998), or from U.S. Pat. No. 6,320,702. The known zoom type comprises, viewed from the object, four optical assemblies G1, G2, G3, and G4, groups G1 and G4 being arranged in stationary fashion. Group G1 possesses a positive focal length f1, group G2 a negative focal length f2, group G3 again a positive focal length f3, and the fourth group G4 once again a negative focal length f4. To modify the magnification of the zoom, the movably arranged groups G2 and G3 are displaced. FIG. 1a) indicates the highest-magnification position, and FIG. 1b) the lowest-magnification position. The change in the position of assemblies G2 and G3 is accomplished, under the control of cams, along optical axis 2. The Wxc3xcllner equations known from the literature can be used to calculate the corresponding distancesxe2x80x94i.e. distance D12 between assemblies G1 and G2, distance D23 between assemblies G2 and G3, and distance D34 between assemblies G3 and G4xe2x80x94on the basis of a known distance between the focal points of groups G1 and G4, the known focal lengths f2 and f3, and a selected magnification (of groups G2 and G3).
As depicted in FIG. 1, D23 is minimal at the greatest magnification and increases from there with decreasing zoom magnification, so that D12 and D34 are minimal at the lowest zoom magnification. The zoom factor of a system of this kind is limited only by the fact that assemblies G2 and G3 at maximum magnification, and assemblies G1 and G2 as well as G3 and G4 at minimum magnification, must not interpenetrate.
ENP designates the diameter of the entrance pupil of zoom 1 at the greatest magnification (FIG. 1a)). Diameter EP of the entry pupil of the zoom is maximal at the greatest zoom magnification. Entrance field angle wE of the zoom designates the visual angle at which an object appears at infinity. This angle becomes minimal at the weakest zoom magnification and assumes a value w1, as is evident from FIG. 1b). Overall length L of the zoom corresponds to the distance between the outer vertices of assemblies G1 and G4.
FIG. 2 is a sketch of a microscope having an afocal zoom 1. An object 9 is arranged at the anterior focal point of objective 10, and is imaged thereby at infinity. The downstream afocal zoom 1 modifies the magnification within a selectable range, and once again images the object at infinity. Arranged behind zoom 1 is a tube lens 11 which generates an intermediate image 12 that in turn is visually observed through an eyepiece 13 by eye 17. EP designates the diameter of the entrance pupil of zoom 1. AP designates the diameter of the exit pupil of the microscope after eyepiece 13. It is known that the resolution of the microscope depends on numerical aperture nA of objective 10, which is defined as the sine of half the angular aperture xcex1 of the cone having its vertex at the center of the object and is limited by entrance pupil EP. Well-corrected optical systems that satisfy the sine condition are known to be governed by the equation EP=2xc3x97fO nA, where fO refers to the focal length of objective 10. For a wavelength xcex=550 nm, the rule of thumb for calculating the resolution capability is 3000xc3x97nA (in line pairs per millimeter). Since the numerical aperture increases with the diameter of the entrance pupil, it is obvious that a large diameter EP is needed in order to achieve high resolution.
FIG. 3 shows the schematic construction of a stereomicroscope of the telescopic type. The stereomicroscope allows the viewer to obtain a three-dimensional impression of object 9 being viewed. For that purpose, object 9, which is located at the anterior focal point of objective 10, is imaged through two separate observation channels. The two observation channels 15L and 15R are of identical construction and each contain a zoom system 1L, 1R, a tube lens 11L, 11R, and a respective eyepiece 13L and 13R. Image erection systems 16L, 16R arranged behind tube lenses 11L, 11R provide right-reading erect intermediate images 12L and 12R which are visually viewed by a pair of eyes 17L and 17R using the pair of identical eyepieces 13L, 13R. The two zoom systems 1L and 1R selectably modify the magnification, but identically for the right and the left channel.
The two intermediate images 12L and 12R are different images of object 9, since object 9 is viewed at angle wL in left channel 15L and at angle wR in right channel 15R. This makes possible stereoscopic viewing of object 9, just as an object is viewed by the pair of eyes. The two different images are processed in the brain to yield a three-dimensional image.
EP once again designates the diameter of the entrance pupil of the zoom, EP being identical for the two identically adjustable zooms 1L and 1R. uL and uR designate half the angular aperture of the cone, with vertex at the center of the object, that is limited by the entrance pupil. uL and uR are identical in size, since the microscope is symmetrical with respect to axis 14 of objective 10. uL and uR can consequently both be designated u. Since wR and wL are not large, the relevant equation (by analogy with the microscope of FIG. 2) is EP=2xc3x97fOxc3x97sin(u)=2xc3x97fOxc3x97nA, where nA once again represents the numerical aperture, but this time referred to the entrance pupil of the zoom, downstream from objective 10, in each channel.
The aforementioned article by K. -P. Zimmer, xe2x80x9cOptical Designs for Stereomicroscopesxe2x80x9d (1998), presents a zoom, for a stereomicroscope as described above, such as the one depicted schematically in FIG. 4. A zoom of this kind was placed on the market by the Applicant on Apr. 3, 1995 under the designation xe2x80x9cMZ 12.xe2x80x9d This zoom is constructed in accordance with the preamble of Claim 1. At the greatest magnification VZO of the zoom, the entrance pupil diameter ENP=20 mm, the distances between optical assemblies G1, G2, G3, G4 are D12=33.53 mm, D23=2.99 mm, and D34=36.82. The focal lengths of the groups are f1=65.47 mm, f2=xe2x88x9215.30 mm, f3=32.17 mm, and f4=xe2x88x9243.65 mm. At its lowest magnification setting (see FIG. 4b)), the distances are D12=7.51 mm, D23=62.82 mm, D34=3.01 mm. The zoom depicted in FIG. 4 has a zoom factor z of 12.5, and |f1/f2|=4.28.
The zoom depicted in FIG. 4 can, in accordance with the Wxc3xcllner equations known from the literature, be adjusted to a greater zoom factor z by displacing optical assemblies G2 and G3 farther along optical axis 2 without interpenetration of the lens groups. As is evident from FIG. 5a), assembly G2 is displaced 0.12 mm toward G3, and assembly G3 is displaced 0.8 mm toward G2, so that distance G23=2.07 mm and accordingly D12=33.65 mm and D34=37.62 mm. At this setting, zoom magnification VZO=52. In FIG. 5b), as compared to the setting in FIG. 4b), assembly G2 is displaced 2.27 mm toward G1, and G3 is displaced 0.85 mm toward G4. At this setting, zoom magnification VZO is 0.3517. This therefore results in a zoom factor z=14.8 for the zoom depicted in FIG. 5. In addition |f1/f2|=4.28 and |z/(f1/f2)|=3.46.
The disadvantage of the configuration depicted in FIG. 5 is that an increase in the zoom range toward higher magnifications does not result in higher resolution. Higher resolution can be obtained only by means of greater entrance pupil diameters, and with stereomicroscopes in particular this results in large dimensions (see Zimmer, xe2x80x9cOptical Designs for Stereomicroscopes,xe2x80x9d 1998, p. 693). Once the resolution limit has been reached, an increase in microscope magnification yields so-called xe2x80x9cempty magnification,xe2x80x9d in which no further details become recognizable despite the increasing magnification.
Lastly, U.S. Pat. No. 6,320,702 B1 protects an afocal zoom for microscopes that also comprises four assemblies G1 through G4 that have alternately positive and negative focal lengths. Claimed therein are zooms having a zoom factor z greater than 14 and a focal length ratio between focal length groups G1 and G2 |f1/f2| greater than 3.9. Cited therein as an advantageous further condition is 3 less than |z/(f1/f2)| less than 5. This limitation is intended to prevent not only excessive zoom lengths, but also any interpenetration of first and second lens groups G1 and G2.
The zoom according to U.S. Pat. No. 6,320,702 B1 contains more lens elements in total than the zoom depicted in FIGS. 4 and 5, and furthermore has the disadvantage, discussed in connection with FIG. 5, that a higher zoom factor alone is not suitable for increasing the resolution of the microscope. In addition, a greater zoom magnification is disadvantageously associated with a reduction in the diameter of the microscope""s exit pupil at maximum magnification.
It is therefore the object of the present invention to describe an afocal zoom of the kind cited initially, for high resolution microscopes having a high zoom factor, which makes possible a continuously modifiable magnification over the greatest possible range simultaneously with the greatest possible resolution. In addition, the requirements in terms of increasing fields of view are to be met.
This object is achieved by way of the features of independent Claims 1 and 2. Advantageous embodiments are evident from the respective dependent claims and from the description below.
The condition VZOxe2x89xa641xc3x97ENP/fT, hereinafter referred to as (B1), defines for the zoom magnification an upper limit that is proportional to the ratio between diameter ENP of the zoom entrance pupil at maximum zoom magnification and focal length fT of the microscope""s tube lens. The performance data of the zoom are thereby linked to optical variables of the microscope. Conformity with (B1) guarantees that the high microscope magnification lies within the range of useful magnification, and limits the contrast falloff with small exit pupils.
The xe2x80x9cuseful magnificationxe2x80x9d of a microscope is defined as that range of microscope magnification within which all object features are resolved, imaged in magnified fashion, and recognized by the human eye. Greater magnification is possible but not useful, since additional details, which cannot be imaged by the microscope objective because of limited resolution, cannot be recognized (i.e. empty magnification, resulting in a larger image but not in finer features). Detail recognition additionally depends on the contrast of the image. The exit pupil diameter of the microscope plays a role here, since with increasing diameter, brighter images are supplied and the contrast loss due to diffraction in the eye and irregularities in the vitreous body of the eye are reduced.
Conformity with condition (B1) according to the present invention thus ensures, for high-resolution microscopes with ENP greater than 21 mm, that as zoom magnification VZO increases, detail recognition also rises, and that the contrast falloff which works against detail recognition at the same time remains limited.
While (B1) defines a condition for the afocal zoom at maximum magnification, condition (B2) below describes a requirement for the zoom at its lowest magnification. According to the present invention, (B2) is tan(w1) xe2x89xa70.268xc3x97z/ENP, in which w1, as is evident from FIG. 1b), is defined as the entrance field angle of the zoom at minimum magnification, and z represents the zoom factor, i.e. the ratio between maximum and minimum zoom magnification, such that z greater than 15. ENP (in units of mm) once again designates the diameter of the zoom entrance pupil at maximum magnification (cf. FIG. 1a)), such that ENP greater than 21 mm. Conformity with B2) guarantees, like (B1), that operation is occurring in the range of useful magnification, and also that at the lowest magnification, a field-of-view diameter (diameter of the intermediate image) of at least 22 mm is usable. This advantageous property for a microscope is stated by condition (B2) on the basis of the requisite performance of the zoom in terms of field angle w1 at minimum magnification, taking in to account zoom factor z and maximum diameter ENP of the entrance pupil. The advantageous result is that in zooms having a given entrance pupil diameter ENP, a larger field of view is usable even as zoom factor z increases. This prevents vignetting at the lowest magnifications, and thus makes possible rapid positioning of specimens and/or an improved overview for inspections.
Simultaneous conformity with conditions (B1) and (B2) is advantageous because this ensures that when working with the zoom according to the present invention, the microscope magnification lies within the range of useful magnification, and a large field of view is available at low magnifications as well as sufficient resolution at the highest magnifications.
It has proven to be advantageous if the diameter of the zoom entrance pupil at maximum magnification satisfies the condition 21 mm less than ENPxe2x89xa627 mm. Such entrance pupil diameters are particularly well-suited in practice for meeting condition (B1). Larger entrance pupil diameters result in practice, especially in the case of stereomicroscopes, in large physical dimensions and also in increasingly severe aberrations. Smaller entrance pupil diameters, on the other hand, result in decreased resolution.
It is additionally advantageous in terms of zoom factor z if the condition 15 less than zxe2x89xa620 is met. In combination with conditions (B1) and (B2), at these zoom factors operation in the range of useful magnification with sufficiently large field of view in the low-magnification region is easily achievable in practice.
The overall length of the zoom is of great importance for both ergonomic and production-engineering reasons. A long zoom means a large overall height for the microscope, and complicates fatigue-free viewing. Large entrance pupil diameters EP and large zoom factors z are difficult to achieve in physically short zooms. In the zoom according to the present invention, it has proven advantageous to impose an upper limit on zoom length L using the following condition (B3):
L/ENPxe2x89xa6kxc3x97{square root over (z)}xe2x89xa61.37{square root over (z)}
where L is the length of the zoom measured between the outer lens element vertices of assemblies G1 and G4, and k is a length factor lying in the range from 1.34 and 1.37. For the embodiments according to the present invention described below, an upper limit using k=1.34 can be observed.
High-resolution microscopes according to the description above require a large zoom entrance pupil diameter ENP at maximum zoom magnification. To allow a short overall length nevertheless to be achieved for the zoom, the construction of assembly G1 should advantageously be such that focal length f1 of assembly G1 remains small despite a large ENP. The following inequality can be stated as a particularly favorable condition (B4):
xe2x80x83f1/ENPxe2x89xa63.5.
Exemplary embodiments of zooms according to the present invention are presented which meet condition (B4) at an upper limit of 3.3 rather than 3.5, and thus advantageously contribute to a short overall length and good imaging performance at large ENPs.
An upper limit on the number of lens elements is advantageous in terms of production engineering, and limits costs. Zooms according to the present invention having a maximum of eleven lens elements are especially favorable in this regard.
It is particularly advantageous in terms of the zoom""s manufacturing costs if assembly G4 comprises a maximum of two lens elements cemented together. It is furthermore advisable if assembly G1 is constructed from a cemented group followed by an individual lens element, the cemented group comprising two lens elements cemented to one another, the individual lens element being biconvex, and the lens of positive refractive power in the cemented group pointing toward the object.
A particularly favorable embodiment of an afocal zoom according to the present invention is described by Table 1 referring to the first exemplary embodiment. This embodiment is described below in more detail.